Phyllotreta Chevrolat in Dejean, 1836
With about 300 species and subspecies and a cosmopolitan distribution this is among the better known and largest of the flea beetle genera, the greatest diversity is in northern temperate regions with about 150 species in the Palaearctic region, of which about 118 are endemic and about 50 in North America, of which 40 are endemic. By contrast 25 species (18 endemic) are known from the oriental region, 5 (3 endemic) from the Neotropical region, 49 (39 endemic) from the Afrotropical region and 4 (3 endemic) to the Australasian region. Thus a high proportion of all regional faunas are endemic but in some cases, especially in North America, several have been introduced with trade from other regions. The European fauna includes 66 species, many of which are either widespread across the region or predominantly south western or eastern in distribution. The greatest diversity is found in the south and it is here that many of the restricted species occur e.g. P. gloriae Biondi, 1994, P. hispanica Pic, 1903 and P. iberica Heikertinger, 1911 are endemic to Spain while P. hemipoda Abielle, 1909 is endemic to Portugal and parts of north western Africa. In general the species vary little and subspecific differentiation is unusual; among the European fauna this is seen only in P. variipennis (Boieldieu, 1859) which occurs continuously from Portugal to Turkey and is also present in North and Tropical Africa and on many of the Mediterranean and Atlantic islands; the nominate subspecies occurs throughout most of the range while ssp. aegyptiaca Pic, 1915 is restricted to warmer regions of Africa, the Near East and the Canary Islands. By contrast a few are restricted to cooler regions e.g. P. zimmermanni (Crotch, 1873) is widespread across North America and the Eastern Palaearctic but is known in the west only from Finland and Sweden. The UK fauna includes 15 otherwise widespread species.
The genus is well known because it includes many brassica pests, some of which continue to be of economic importance in northern temperate regions. The genus is generally adapted to these hosts (at least in terms of dealing with the chemical defences etc of the plants); host plants of most Neotropical, Afrotropical and Australian species are unknown but of the 117 temperate species for which host plants are known the majority are specialist feeders on Brassicales i.e. they are able to deal with the glucosinolates defences of the plants, many of these are oligophagous on brassicas and closely related families and only very few are known to attack plants which do not synthesize glucosinolates e.g. the Southern European and African P. cruralis Abeille, 1895 is a specialist feeder on various Amaranthaceae. In general more than 60% of species are specialist Brassicaceae feeders, about 18% feed on Brassicas as well as certain other families including Capparaceae, Cleomaceae, Resedaceae and Tropaeolaceae, and a small number are oligophagous on a range of other families including Asteraceae and Malvaceae (Gikonyo, Biondi and Beran, 2019). The chemical relationships between Phyllotreta species and their host plants is complex and very varied but at least some e.g. P. striolata (Fabricius, 1801) are able to utilize sequestered glucosinolates for their own defence against predators. The life history is typical of the tribe with immature adults overwintering in soil or litter etc. and active from early spring until the summer when they generally die off and are replaced by the new generation. There is evidence that some species in Europe may also overwinter in the soil as eggs and that larval development begins in early spring but this seems to be atypical. Wintering sites are often in sheltered situations away from host plants, and in arable situations mass migrations by flight may occur when conditions are favourable. Once active they do not survive long without feeding and so uncultivated brassicas may be attacked until crops are sufficiently developed to host them, a feature of some species is that they can feed as adults on non-host plants but this is an emergency measure and it is likely that normal adult maturation may not be possible without brassica hosts. Breeding occurs in spring and early summer following a period of maturation feeding which may last up to 5 weeks, and females generally lay small batches of eggs in the soil around host roots or, in a few cases, occasionally on host foliage. The eggs are elongate, between 0.5 and 0.9 mm, pale yellow and translucent, and in most cases under normal conditions they hatch within two or three weeks. The creamy or yellowish larvae develop either in the soil and feed externally on roots or they mine into roots and feed internally although larvae of some e.g. P. undulata Kutschera, 1860, may feed externally on leaves until they are fully-grown. As far as is known all species pupate in a silken cocoon in the soil from 1 to 12 cm deep and in most cases this stage lasts between 1 and 3 weeks. A typical development time from egg to adult is 30 to 50 days and in northern temperate regions there is usually a single generation each year. Several species are serious pests of cultivated brassicas e.g. P. cruciferae (Goeze, 1777) and P. striolata cause damage to oilseed rape in Canada which is estimated at tens of millions of dollars. In the UK some species may be common among rape and other crops but they are more of a nuisance than a serious pest.
Adults are likely to occur during the warmer months by general sweeping, especially on arable borders but also generally, they often appear in winter extraction samples and occasionally in flood refuse. Placing yellow pan traps among brassica crops is a good way to sample them, they generally occur in numbers during warm weather and often along with other leaf-beetles such as Psylliodes chrysocephala (Linnaeus, 1758). Yellow-pan traps are the basis of many commercial control traps used against brassica pests, especially Brassicogethes aeneus (Fabricius, 1775). They are deployed when pest numbers reach a certain level and they can be brutally effective, especially when armed with semiochemical lures based on various plant volatiles, killing many thousands of the target species as well as large numbers of other brassica-feeding insects.
Phyllotreta are small; less than 5 mm but mostly between 2.5 and 4.0 mm, elongate and depressed species, the body is dark, glabrous and shiny, either entirely dark and with or without a metallic reflection or with a longitudinal yellow stripe along each elytron which is sometimes interrupted at the middle to produce a four-spotted appearance. In a few foreign species the elytra are entirely pale but for a narrow dark sutural line. The appendages are usually dark although the basal antennomeres are often lighter and the tibiae and tarsi may be partly or wholly yellow, in rare cases e.g. the widespread European P. armoraciae (Koch, 1803) the legs may be almost entirely yellow but for the hind femora which are dark in all species. Head with large and convex eyes and strongly converging frontal furrows, surface at least partly punctured and often with distinct cellular microsculpture, clypeus broad and usually straight anteriorly, labrum weakly curved anteriorly. Antennae 11-segmented and filiform or, in some males, with the fourth and/or fifth segments enlarged. Pronotum transverse, broadest at or behind the middle and narrowed to distinct posterior angles and an elongate lateral tubercle behind slightly projecting anterior angles. Surface evenly and weakly convex; not depressed in front of the base and without latero-basal impressions, punctation and microsculpture very variable. Elytra elongate and curved from rounded or angled shoulders to a continuous apical margin, surface randomly punctured throughout although in some they are seriate across the disc, microsculpture very variable. Hind femora greatly enlarged, front and middle femora normal. Tibiae smooth externally, hind tibial spur placed medially on the lower margin. Tarsi pseudotetramerous, in males the front and middle tarsi are dilated but this is often weakly developed and only obvious in series. Our UK species might be confused with Aphthona but here the hind tibial spur is placed at the edge of the apical margin, in any case Aphthona are more convex and usually less elongate and will soon be appreciated with a little experience, our yellow-striped species might be mistaken for Longitarsus dorsalis (Fabricius, 1781) but here the pronotum is pale and the elytral stripes are lateral. Dark species of Longitarsus can always be distinguished from Phyllotreta by the long basal segment of the hind tarsi.